The present invention relates to a method and apparatus for controlling valves, and more specifically to a method and apparatus for adjusting the timing of internal combustion engine poppet valves.
It has been known for some time that throttling losses contribute significantly to poor engine efficiency at light power levels. In 1958 the Society of Automotive Engineers (SAE) published “Determination of True Engine Friction,” SAE Trans., Vol. 66, pp. 649–661 which deals with throttling and aerodynamic pumping losses. Throttling losses remain a significant problem with spark-ignition passenger cars, as the throttle remains the principal method of regulating engine power output.
German automobile manufacturer BMW announced that it has developed a spark ignition engine with no throttle, and having power output controlled by an adjustable valve control apparatus, which is described in Automotive Engineering International, pp. 106, May 2001. According to BMW, the engine will be the first spark-ignited automotive engine ever offered for sale without a throttle. Engine power is controlled by adjusting the lift and timing of the intake valves. A problem with the BMW mechanism is that the intake valves restrict (e.g., throttle) air flow at the intake ports, and as a consequence pumping losses are only partially reduced relative to conventional throttled engines. Another significant problem with the BMW mechanism is that of cost. The system reportedly will increase engine cost by about 15 percent. Another problem with the mechanism is that it increases engine height and cannot be easily packaged into a broad range of automobile makes and models.
R. J. Saunders, T. H. Ma, and others show an Otto Atkinson cycle engine in SAE paper 910451 (pub. 1991) and in IMechE paper 925107 (pub. 1992) which employs adjustable valve timing to control engine power. Specifically, each cylinder has two intake valves, the timing of one of the two valves having an adjustable phase timing to control the time at which the cylinder is sealed and compression started. At light power levels, the phase shifted intake valve is closed late, causing much of the intake air to be pushed back into the intake manifold by the piston before the start of charge compression within the cylinder. The late intake valve closing is effective for controlling engine power and almost fully eliminating throttling and aerodynamic pumping losses.
With regard to the valve control mechanism shown by Saunders et al., each cylinder has two exhaust valves and a first intake valve driven by a first camshaft, a second intake valve driven by a second camshaft, and a phase adjuster for adjusting the phase relationship between the first and second camshafts. The exhaust valves and the second intake valve have inverted bucket type tappets that are driven by the respective camshafts. The first intake valve is driven by a rocker arm that reaches across the cylinder head from the first camshaft to the first intake valve, the first camshaft being located directly above the exhaust valves. The camshaft bearings for the first camshaft are located between the adjacent exhaust valves of each cylinder, and the intake rocker arm cam lobes are located immediately adjacent to one another for adjacent cylinders, providing a total of four cam lobes between some cam bearings. A significant problem with the valve adjustment mechanism is that the rocker arms are long and massive, adversely effecting mechanical friction and engine top end speed. Another problem is that the camshaft is prone to bending, resulting in valve train wear, due to the large cam lobe count between camshaft bearings, and further with the most highly load rocker cam lobes being located mid-span between the bearings. Another problem with the system disclosed by Saunders is that the valve lash will require frequent servicing due to the massive rockers in combination with the assembled multi-piece rocker mounts. In contrast, automotive manufacturers are now producing valve trains that do not require service for 100,000 miles. A further problem with the system disclosed by Saunders is that of cost due to the rocker mount assemblies and due to difficulty in establishing alignment from cylinder to cylinder.
A principal limitation of modern spark-ignition engine power output is that of limited space available for the intake and exhaust ports, resulting in small and non-streamlined ports, resulting in limited airflow into the cylinders and limited exhaust flow out of the cylinders. Additionally, high power output cylinder heads require a greater cooling capacity water jacket above the combustion chamber, which requires additional space. Increasing the outward size of the cylinder head does not substantively improve port size immediately above the valves where flow is most restricted, and adversely effects cylinder head weight and overall size, a small size being important for packaging the engine into the engine bay of the vehicle. In general, space limitations within the cylinder head prevent use of optimum size, shape and placement of the intake and exhaust ports, and also prevents the simultaneous optimum placement of the water jacket, spark tube, valve springs, tappets and/or rockers, camshafts, camshaft bearing caps, bearing cap fasteners, head bolts, and other cylinder head parts.
The problem of compromised cylinder head design resulting from space constraints is exacerbated in engines having two intake valves per cylinder, where one of the two intake valves has an adjustable phase or adjustable timing to control engine power. As one example of the adverse effect of limited space within the cylinder head, in the system described by Saunders et al., space is not available between the intake rocker arms for a single rocker shaft mount, and consequently two rocker shaft mounts are required, the two rocker shaft mounts being more expensive. Additionally, the two rocker shaft mounts occupy the space inboard of the exhaust valves, precluding and/or complicating use of inboard exhaust rockers located between the exhaust and intake valves. Additionally, space is not available between the rockers for a camshaft bearing, and consequently the camshaft is under supported. Further, the cylinder head bolts are located approximately inline with the exhaust camshaft, and largely concealed by the intake rockers.
Accordingly, objectives of the present invention include near-elimination of throttling and aerodynamic pumping losses, large high flow intake and exhaust ports, a high cooling capacity water jacket, no or almost no increase in mechanical friction, a high maximum engine speed capability, no or almost no increase in engine size, high durability and infrequent required service, and a low cost.